This invention relates to a method for preparing catalysts useful in the oxidation of hydrocarbons. More particularly it is directed to the preparation of catalysts suitable for the production of dicarboxylic acid anhydrides from hydrocarbons, such as the production of maleic anhydride from 4-carbon atom hydrocarbons, such as n-butane, n-butenes, 1,3-butadiene or a mixture thereof.
Catalysts containing vanadium and phosphorus oxides have been used in the oxidation of 4-carbon atom hydrocarbons, such as n-butane, n-butenes, 1,3-butadiene or mixtures thereof with molecular oxygen or oxygen-containing gas to produce maleic anhydride. Conventional methods of preparing these catalysts involve reducing a pentavalent vanadium compound, and combining the same with a phosphorus compound and, if desired, promoter element compounds under conditions which will provide or maintain vanadium in a valence state below +5 to form catalyst precursors capable of being converted to an oxide. The catalyst oxide precursor is then recovered and calcined to provide active catalytic material.
U.S. Pat. No. 3,985,775 describes the preparation of vanadium phosphorus catalysts in an aqueous solution of concentrated hydrochloric acid. This patent also describes the catalyst preparation in non-aqueous media, where corrosive reducing agents were added to the media or were generated in situ.
The use of gaseous HCl as a reducing agent for vanadium is disclosed in U.S. Pat. No. 4,002,650 where the vanadium and phosphorus components are reacted in an aqueous solution. The use of gaseous HCl as a reducing agent for vanadium is also described in U.S. Pat. No. 4,043,943 where the vanadium and phosphorus components are reacted in a liquid organic medium.
U.S. Pat. No. 4,016,105 describes the preparation of vanadium and phosphorus oxide-containing catalysts, utilizing as reducing agents, organic acids or aldehydes, together with a co-reducing secondary alcohol. These reducing agents are added to an aqueous solution with the vanadium and phosphorus components.
Similar preparational techniques are described in European Patent Application No. 3,431 in which the additional step of comminuting the vanadium-phosphorus precursor to a particle size of 500 to 700 microns (0.5 to 0.7 mm) is disclosed.
The use of such reducing agents as disclosed in the art requires special precautions in the preparation of these catalysts because of the corrosive nature of the materials utilized. The process of the present invention permits the preparation of mixed vanadium phosphorus oxide catalyst without the use of corrosive reducing agents.
A method for preparing catalysts containing vanadium and phosphorus oxides was described in U.S. Pat. No. 4,132,670, which required the maintenance of a solid phase and dispersion of the vanadium-containing feed compound. The method includes forming a vanadium-containing compound dispersion in an organic liquid medium such as alcohols, aldehydes, ketones, ethers or mixtures thereof, heating the dispersion to reduce the vanadium, and thereafter adding phosphoric acid in an organic solvent.
The preparation of oxidation catalysts containing the mixed oxides of vanadium and phosphorus is disclosed in U.S. Pat. No. 4,244,879 wherein a vanadium compound is at least partially solubilized in an organic liquid medium capable of reducing at least a portion of the vanadium to a +4 valence state, and unsolubilized vanadium having a particle size larger than about 0.1 mm diameter is removed from the medium before addition of a phosphorus-containing compound.
The preparation of vanadium phosphorus mixed oxide containing catalysts is disclosed in co-pending U.S. Ser. No. 146,971, assigned to our common assignee, wherein partial reduction of a pentavalent vanadium compound is effected in the presence of a phosphorus compound in an organic liquid medium capable of reducing the vanadium.
Co-pending U.S. Ser. No. 220,629, now U.S. Pat. No. 4,361,501, also assigned to our common assignee, discloses the preparation of vanadium phosphorus oxide catalysts utilizing a mixed phosphorus component compound source, preparing the catalyst in a liquid medium capable of reducing the vanadium component.
Generally, the preparation of vanadium phosphorus mixed oxide containing catalysts in organic media rather than aqueous media, resulted in the production of catalysts having higher activity for oxidation reactions, such as the oxidation of n-butane to produce maleic anhydride. The organic-derived vanadium phosphorus mixed oxide catalysts additionally exhibit higher intrinsic surface area than the aqueous-derived vanadium phosphorus mixed oxide catalysts. Although the use of organic preparative media capable of properly reducing the vanadium component of the catalyst alleviated the need to employ corrosive reducing agents with their attendant costs and difficulties, the commercial use of a total organic preparation would be less desirable than a method of preparation which limited the use of organic materials to critical processing steps.